1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier. More specifically, the present invention relates to improved technology of a fixing device adopting an induction heating system.
2. Description of Related Art
An image forming apparatus has a fixing device for heat-fixing a toner image on a recording medium. In the image forming apparatus, the fixing device adopts an induction heating system for realizing energy-saving by remarkably shorting a warming-up time or by reducing consumption energy during standby.
The fixing device using the induction heating system comprises a conductive heating roller where a magnetic flux generating unit using the induction heating system is provided as a heat source, and a pressure roller for forming a nip portion in pressure contact with the heating roller.
The induction heating magnetic flux generating unit comprises a horizontally long core that extends in an axial direction of the heating roller and an induction coil that is wound around this core. An prior magnetic flux generating unit is formed to a length corresponding to a maximum width of a recording medium in the axial direction, in which the whole heating roller (the maximum width) is heated during a fixing operation. Therefore, when passing large-sized recording paper after passing small-sized recording paper, heat absorption by the recording paper is not performed at a portion corresponding to a non-paper passing portion formed at the previous paper passing (when passing the small-sized paper), and heating is performed while keeping the state. As a result, the following problems occur. That is, nonuniformity of a fixing temperature is caused in the axial direction of the heating roller and fixing failure such as hot off-set occurs due to a high temperature. Further, malfunction detection stop of machines is caused due to abnormal temperature rise. For solving the problems, a method for arranging an induction heating magnetic flux generating unit by dividing the unit into a plurality of pieces is heretofore disclosed (see, e.g., JP-Tokukai-2002-351240A and JP-Tokukai-2003-255731A).
JP-Tokukai-2002-351240A discloses an induction heating fixing device comprising a magnetic material core and an induction coil that is divided into a plurality of pieces in a direction vertical to a paper passing direction. In the device, ends of the induction coils are inclined at a predetermined angle with respect to a direction vertical to a recording paper passing direction, whereby an electric current is allowed to pass through all of the divided induction coils to heat the heating roller.
Further, JP-Tokukai-2003-255731A discloses an induction heating fixing device as described below. That is, inside a hollow heating roller, an internal induction coil is arranged throughout the axial direction of the heating roller so that the coil is prevented from being brought into contact with the roller. Outside the heating roller, an external induction coil is arranged throughout the axial direction of the heating roller so that the coil is prevented from being brought into contact with the roller. Further, both of the induction coils are arranged so that the directions of the induction coils are changed to allow magnetic fluxes generating through both the induction coils to be shifted from each other by 90°.
However, according to the prior development as described above, when the induction coils adjacent to each other in the axial direction of the heating roller are brought into contact with each other, magnetic fluxes generating through each induction coil pass throughout the axial direction of the heating roller. As a result, the whole heating roller is always heated. Accordingly, an interval for separating the respective adjacent induction coils from each other must be provided. However, when providing the interval, a portion having no induction magnetic field, namely, a non-heating portion is formed on the heating roller. In the non-heating portion, temperature drop is caused. Accordingly, there arises a problem that a temperature distribution in the axial direction of the heating roller is made nonuniform, and as a result, fixing failure is caused.